Outline

Background: There is an intense interest in proton beam therapy (PBT) worldwide due its unique physical characteristics providing superior clinical outcome. However an enormous capital requirement in PBT is a major obstacle for its widespread use. The major cost of PBT includes cyclotron/synchrotron that is energy dependent, treatment gantries and associated shielding. An optimum choice of proton energy based on depth of target in our patient population, usage distribution of gantries and snout size is analyzed for a proton facility.

Material and methods: The frequency distribution of the target depths for 105 patients in our conventional external therapy and 218 patients treated at our proton facility was analyzed. A margin of 1.5 cm was added for all treatments for the estimation of the proton energy. Additional data for the disease sites, snout size, and the beam angle utilization in our proton therapy was analyzed.

Results: At MPRI only 52% patients are treated for prostate cancer with opposed lateral fields. The remaining patients are distributed among other disease sites that required non 90–270Â° gantry angles. Figure 1 [Fig.Â 1] shows a wide frequency distribution of the treatment depths related to proton energies with a maximum at about 14 cm. Depth distribution admittedly is biased for proton therapy due to selection of prostate patients. The cumulative depth distribution shows that proton energy of 208 MeV as at MPRI is sufficient to treat 95.6% of patients and 200 MeV is sufficient to treat 95% of patients. The utilization of gantry angles in Figure 2 [Fig.Â 2] shows that the 90–270Â° beam angles has a 46.2% usage which is very close to our prostate patients (52%). Thus the use of a fixed beam line is adequate for prostate treatments. The distribution of snout usage was 76.5%, 18.5% and 5.3 % for the 10, 20 and 30 cm snouts respectively. Hence 95% of patients can be treated without the 30 cm snout and the remaining 5% can be treated with patching fields. Such analysis will also depend on the physique of the patient population in a specific center.

Conclusions: The cost of PBT depends largely on the maximum beam energy and the choice of gantry versus fixed beam line. Our study indicates that for a 4-room center, only two gantry rooms are needed, thus significantly reducing the cost. In the USA, 95% of patients can be adequately treated with 200 MeV proton beam. Increasing the beam energy increases the machine and associated shielding cost and leads to increased secondary neutrons and radioactivity both in the gantry and the patient.